DESCRIPTION (Applicant's Description) Genomic instability is a hallmark of cancer cells. Defects in multiple cellular processes, including DNA repair and cell cycle checkpoint control, can give rise to this phenotype. Recent studies have provided evidence for a link between aberrant DNA double-strand break (DSB) repair and inherited forms of cancer. The overall goal of this program project is to elucidate the molecular mechanisms of DSB repair and to examine the regulation of this process by the products of the tumor suppressor genes BRCA1, BRCA2 and ATM. We hypothesize that abnormalities in DSB repair, which may be in the DNA damage sensing mechanisms, signal transduction responses, or in the repair process itself, are an important causative factor in many human cancers. This research program combines biochemical, cellular, and genetic approaches to delineate the assembly and mechanism of action of DSB repair machinery, how tumor suppressor gene products modulate cellular responses to DNA damage, and how inactivation of tumor suppressor genes results in genomic instability, leading to cancer formation. The individual projects will study: (1) the initiation of DSB repair including break processing by the Mre11/Rad50/p95 nuclease and heteroduplex formation by Rad51 and associated proteins; (2) the DNA synthesis and ligation steps that are required for the completion of DSB repair; (3) the modulation of the DSB repair machinery by an ATM-mediated kinase cascade; (4) the modulation of the DNA repair machinery by BRCA1, BRCA2, and two novel protein factors MLB1 and RLB1. The research projects will be supported by core facilities for animal studies, nucleic acid and protein analyses, and imaging, along with an administrative core. These highly integrated projects will interact synergistically to enhance our knowledge of a complex multi-component DNA repair system that plays a critical role in the maintenance of genomic stability. Results from these studies will form the basis for new strategies in cancer detection, prevention, and treatment.